1. Field of the Invention
The present invention relates to nonlinear optical chromophores and, more particularly, pertains to highly active second-order nonlinear optical chromophores with reduced degree of trans-cis isomerization and devices incorporating the same.
2. Description of the Related Art
Organic second-order nonlinear optical (NLO) materials have received increasing attention for applications involving signal processing and telecommunications. One of the challenges in this field is to design and synthesize second-order NLO chromophores (the active components of second-order nonlinear optical materials) that simultaneously possess large first molecular hyperpolarizabilities (xcex2), good chemical, thermal and photochemical stability, and optical transparency at optical communication wavelengths (1.3 and 1.55 xcexcm). Chromophore intermolecular electrostatic interactions prevent the simple scaling of molecular optical nonlinearity into macroscopic optical nonlinearity. Such interactions strongly attenuate the efficient induction of acentric chromophore order (hence, electrooptic activity) by electric field poling or self-assembly methods. Chromophores with xcex2 values many times those of the well-known Disperse Red 19 dye are thus required to obtain electrooptic coefficients comparable to or higher than those of the leading commercial material crystalline lithium niobate.
The value of xcex2 for a chromophore can be increased by using a diene moiety in place of thiophene in the conventional phenylethenylenethiophene xcfx80-conjugated bridge. Moreover, this enhancement in xcex2 can be accomplished without an increase in the wavelength of the charge-transfer absorption xcexmax. However, the resulting phenylpolyene bridge has poor thermal stability unless the polyene structure is sterically protected (ring locked).
In addition to microscopic and macroscopic nonlinearity, the chemical stability and alignment stability of second-order NLO material are also major problems which must be solved for successful employment of these materials in commercial devices. Chemical degradation of the material are caused by photoinduced chemical reaction and thermal decompostition. In oxygen-containing environment (e.g. air) photoinduced oxidation of chromophore is the major cause of chromophore degradation. Photoxoidation changes the chromophore to a new species that is effectively electrooptically inactive.
Orientational relaxation is also a major problem. The loss of chromophore dipole alignment is caused by photo-induced or thermally-induced structural isomerization, thermodynamic randomization and interchromophore electrostatic interaction, which favor a centrosymmetric antiparallel arrangement of dipoles. The dominant mechanism of photodegradation of chromophores in an oxygen containing environment is photo-oxidation by oxygen.
Properties (microscopic nonlinearity, macroscopic, chemical and thermal stability, etc) of second-order nonlinear optical material are inter-related. Optimization of one property often causes attenuation in other properties. A systematic approach to addressing both the stability and nonlinearity issues is needed for a balanced improvement of both properties.
The nonlinear optical devices and chromophores of the present invention address both the stability and nonlinearity issues, and embody a systematic approach to obtaining a balanced improvement of both properties.
According to exemplary preferred embodiments of the present invention, a solution to the dipole stability problem is to modify the chromophore structure to reduce the amount of (or completely eliminate) the structural units that are potential sources of randomization and to add some structure feature that reduces interchromophore dipole interaction.
The present invention provides for improvements in the chemical and alignment stabilities of polyene-bridged chromophores. It has been observed that polyene-bridge systems that contain free carbon-carbon double bonds can undergo trans-cis isomerization under radiation of light or when subjected to elevated temperatures. The trans-cis isomerization leads to low chemical stability (low decomposition temperature) and causes randomization of chromophore noncentrosymmetric alignment. According to the present invention, reducing the amount of freely isomerizing double bonds in bridge structures provides an effective way to enhance both the chemical stability and the alignment stability of electrooptic materials.
It has been observed, by studying the photochemical stability of highly active (high xcexcxcex2) chromophores in inert atmospheres, that the removal of oxygen greatly enhances the photochemical stability of electrooptic (EO) materials and the complete elimination of oxygen from the device and the atmosphere is a solution to the problem of oxygen-related photochemical degradation. According to an exemplary preferred embodiment of the present invention, an electrooptic device is hermetically packaged in a container filled with inert gas.
A variety of different molecular structures are possible for the chromophores of the present invention. An exemplary preferred class of chromophores according to the present invention includes an electron donor group, an electron acceptor group and a ring-locked bridge structure therebetween, with the bridge structure being directly connected to the electron donor via a single bond. In this class of chromophores, there are two carbon-carbon double bonds that can undergo trans-cis isomerization. In a preferred embodiment, the bridge structure also includes at least one bulky side group to reduce interchromophore dipole interaction.
Another exemplary preferred class of chromophores according to the present invention includes an electron donor group, an electron acceptor group and a ring-locked bridge structure between the electron donor group and the electron acceptor group, with two free double bonds, one located between the donor and the bridge and the other located between the (fused) ring bridge and the acceptor. In a preferred embodiment, the bridge structure also includes at least one bulky side group.
Another exemplary preferred class of chromophores according to the present invention includes an electron donor group, an electron acceptor group, and a bridge structure therebetween, with the chromophores having no carbon-carbon double bond between the donor and the (fused) ring bridge. In this class, there is only one unlocked carbon-carbon double bond between the (fused) ring bridge and the acceptor. In a preferred embodiment, the bridge structure also includes at least one bulky side group.
Another exemplary preferred class of chromophores according to the present invention includes an electron donor group, an electron acceptor group, and a ring-locked bridge structure therebetween, with a built-in electron-withdrawing cyano group on the last ring of the (fused) bridge. In a preferred embodiment, the bridge structure also includes at least one bulky side group.
Another exemplary preferred class of chromophores according to the present invention include any electron donor group, an electron acceptor group including a linear conjugated triene bearing four cyano groups (e.g., new electron acceptor group 4CF or 4CI disclosed herein), and any bridge structure therebetween.
The NLO materials of the present invention are suitable for a wide range of devices. Functions performed by these devices include, but are not limited to, the following: electrical to optical signal transduction; radio wave to millimeter wave electromagnetic radiation (signal) detection; radio wave to millimeter wave signal generation (broadcasting); optical and millimeter wave beam steering; and signal processing such as analog to digital conversion, ultrafast switching of signals at nodes of optical networks, and highly precise phase control of optical and millimeter wave signals. These materials are suitable for arrays which can be used for optical controlled phased array radars and large steerable antenna systems as well as for electrooptical oscillators which can be used at high frequencies with high spectral purity.